1. Field of the Invention
Generally, the present disclosure relates to integrated circuits and methods for the formation thereof, and, more particularly, to integrated circuits including diffusion barriers and methods for the formation thereof.
2. Description of the Related Art
Integrated circuits typically include a large number of circuit elements which include, in particular, field effect transistors. The circuit elements in an integrated circuit may be electrically connected by means of electrically conductive metal lines formed in a dielectric material, for example, by means of damascene techniques. The electrically conductive metal lines may be provided in a plurality of interconnect layers that are stacked on top of each other above a substrate in and on which the circuit elements are formed. Metal lines in different interconnect layers may be electrically connected with each other by means of contact vias that are filled with metal.
For providing a relatively small resistivity of the electrically conductive metal lines, which may allow a reduction of a cross-sectional area of the electrically conductive metal lines, the electrically conductive metal lines may be formed of a metal including copper, for example, substantially pure copper or an alloy including copper and one or more other elements such as, for example, aluminum. However, a diffusion of copper from the electrically conductive metal lines into other portions of the integrated circuit can adversely affect the functionality of the integrated circuit. In particular, a diffusion of copper into semiconductor materials such as silicon can adversely affect the semiconductor properties thereof.
In order to substantially avoid or at least reduce the diffusion of copper, diffusion barriers may be provided between the electrically conductive material including copper and the dielectric material wherein the electrically conductive metal lines are provided. Wu et al., “Effects of Nitrogen Plasma Treatment on Tantalum Diffusion Barriers in Copper Metallization,” Journal of the Electrochemical Society, 150(2), G83-G89, 2003, discloses diffusion barrier layers that are formed of tantalum, tantalum nitride or nitrogen plasma treated tantalum.
In other examples, diffusion barriers may include a layer of tantalum, which may be provided on a layer of tantalum nitride. For forming such diffusion barriers, the tantalum nitride layer may be deposited, for example, by means of a technique of physical vapor deposition, such as sputtering. Thereafter, substantially pure tantalum may be deposited on the tantalum nitride layer by means of physical vapor deposition.
Tantalum deposited by means of physical vapor deposition can exist in two different crystal structures, which are denoted as alpha phase tantalum and as beta phase tantalum. In the alpha phase, the tantalum has substantially a body centered cubic (bcc) crystal structure and a relatively small electrical resistivity in a range from about 15 μΩ·cm to about 60 μΩ·cm. In the beta phase, the tantalum has substantially a tetragonal crystal structure and a relatively high resistivity in a range from about 170 μΩ·cm to about 210 μΩ·cm. When depositing a tantalum layer by means of physical vapor deposition, it may be desirable to use a relatively low deposition temperature to reduce the thermal budget required for the tantalum deposition process. Moreover, it may be desirable to use a relatively high ion flux and a relatively high ion energy to accelerate the deposition of the tantalum layer. Under such conditions of the deposition process, a tantalum layer that is substantially formed of beta phase tantalum is typically obtained so that the tantalum layer has a relatively high resistivity.
In typical diffusion barriers including a layer of tantalum nitride and a layer of tantalum, the tantalum layer is typically from about two times to about three times thicker than the tantalum nitride layer. Therefore, the tantalum layer can contribute significantly to the electrical resistance of the diffusion barrier. Since the diffusion barrier is typically provided at the bottom of contact vias, where an electrical contact between the electrically conductive material in the via and the electrically conductive material in an electrically conductive line below the via is made, diffusion barriers including beta phase tantalum can significantly contribute to the resistance of contact vias. Additionally, portions of the diffusion barrier at bottom and sidewall surfaces of trenches filled with an electrically conductive material can increase the electrical resistance of electrically conductive lines when the diffusion barrier includes a substantial amount of beta phase tantalum. Furthermore, known diffusion barriers including a layer of tantalum nitride may have a relatively small step coverage.
In view of the above-mentioned issues, the present disclosure provides methods and semiconductor structures that may help to substantially overcome or at least reduce the above-mentioned issues.